1. Field of the Invention
This invention relates generally to comparators and more particularly to CMOS comparators having a fast response and a stable switch point.
2. Description of the Prior Art
Comparators are typically limited in many applications by having a slow output slew rate in response to an input voltage. In other words, the rise time of most comparators is often too slow. Many circuit applications require a comparator whose response is in the nanosecond range and cannot tolerate a rise time which is in the microsecond range.
Commonly, comparators which have attempted to improve the response time and speed have had various disadvantages. Because of variations in silicon content and other practical manufacturing problems, no two semiconductors are exactly identical. As a result, supposedly identical transistors will have varying parameters commonly called process variations. One disadvantage of previous fast comparators is the sensitivity of the output signal and rise time to variations in both processing and operating temperature. Process variations are particularly troublesome for one-shot circuits. One-shot circuits only produce a single output pulse in response to an input signal exceeding a reference voltage level. An example of a one-shot circuit is a circuit having two comparators with different switch points where a switch point is defined as the value of the input voltage which causes the level of the output voltage to change. Such one-shot circuits derive the output pulse width from the difference between the two switch points. The problem with fast one-shot circuits which utilize different switch points of two fast comparators is that the switch points tend to move in opposite directions over temperature changes and processing variation thereby causing a large variation in the output pulse width. Another disadvantage with some previous comparators used in one-shot circuits is the dependence of the output pulse width on the width of a trigger pulse used to produce the output pulse. A further disadvantage with some previous fast comparator circuits is that the input comparison voltage is coupled to a current electrode of a transistor. Comparators having this structural configuration draw large amounts of current from the input voltage circuit and may create an excessive load on the input circuit. Yet a further disadvantage with some previous fast comparator circuits is that the range of the reference voltage is limited and cannot be any closer to the supply voltage than the sum of the threshold voltages of transistors used in an inverter portion of the circuits.